[TenTec] Orion - K3NA transmit audio chain article now onTen-Tecwebsite

Sinisa Hristov shristov at ptt.yu
Wed May 3 13:38:46 EDT 2006


Quoting from page 7:

   Amplifier linearity check

   Since you have made the transmitter adjustments at the full
   power level appropriate for your class of entry, the monitor receiver’s
   audio has included any impairment introduced by the
   power amplifier (if used).

   An oscilloscope can provide a quick check for linear operation
   of an amplifier. Wind a length of hook-up wire around the coax
   line connected to the amplifier input and another length around
   the output coax. Increasing the number of turns will increase the
   signal picked off the coax; I find this simple approach picks up
   enough signal to deflect the scope beam. Connect one wire to the
   scope’s X input via a probe, and the other wire similarly to the Y
   input. Adjust the X and Y gain controls so that, while transmitting
   the recorded message, a diagonal line of around 45° appears on
   the scope.

   Examine this line, focusing particularly on modulation peaks. If
   the diagonal line remains straight and the audio quality is good in
   the monitor receiver, the amplifier is operating linearly (good!). If
   the tip of the diagonal line starts to deviate away from straight,
   even just a little amount, then the amplifier has moved into nonlinear
   operation. You’re definitely transmitting some garbage on
   the bands then!


It's difficult to imagine a more unreliable way of sampling
amplifier's input and output signals.

A wire wound around coax is a capacitive probe picking an
undeterminate mixture of coax leakage and common mode voltage.

The later is likely to dominate, even on the "input" probe,
invalidating the test because it originates mostly from the
amplifier output (i.e. coax - antenna junction).


73,

Sinisa  YT1NT, VE3EA


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